This application relates to wells which use subsurface pumps to produce fluids from a subsurface reservoir to the surface, where the subsurface pump is actuated by an actuation string, such as a string of sucker rods or a string of small diameter tubing. Typically reservoir pressure declines over time and the wells require some form of artificial lift. This may be true for water wells as well as oil wells.
Sucker rod pumps are the most common form of artificial lift for oil wells. Sucker rod actuated pumps may be operated by reciprocation of the rod string, as with a rod pump having a plunger which seals within a barrel, drawing fluid into the barrel on the upstroke. The rod string is typically reciprocated up and down by a pumping unit at the surface. Another type of subsurface pump—the progressive cavity pump—is actuated by the rotation of the rod string, which rotates a rotor within a stator. In both cases, the rod string acts as a pump actuation string. While the typical installation utilizes a string of individual rods coupled in an end-to-end configuration, it is to be appreciated that other configurations of a pump actuation string may be utilized, such as a string of small diameter tubing joints connected together. In addition, because continuous lengths of tubing or rod are known, such as reeled tubing, etc, such a continuous single length may also be utilized as a pump actuation string.
Pumping units for rod pumped wells must be sized according to the loading induced on the pumping unit by the loading at the polished rod or top rod in the rod string. The sizing is further complicated for wells produced with reciprocating rod pumps because the loading varies as the rod string moves between the up stroke and the down stroke. During the up stroke, the polished rod lifts the fluid, rod string suspended from the polished rod, and the pump plunger. During the down stroke, the polished rod lowers the rods and pump plunger. Therefore the pumping unit would be fully loaded on the upstroke and lightly loaded on the down stroke. Compensation for this uneven loading is partially addressed by counter balance effects (CBE) which attempt to reduce equipment loading and balance the loading across the pumping cycle. In the ideal installation, the CBE would equal the rod weight plus one-half of the fluid load, that is the pumping unit would lift half the fluid on both the up and down stroke. The rod load would be eliminated by the ideal CBE.
The walking beam pumping unit is the most common sucker rod pumping system. These types of units convert rotational motion of the prime mover of the pumping unit into a generally vertical reciprocating motion of the polished rod. Typical pumping units have a horse head which is shaped such that the polished rod reciprocates vertically without a horizontal motion. Therefore the polished rod forces act vertically. This shape produces an increase in the effective length of the waking beam as the walking beam reaches the top and bottom of the stroke. Therefore the horizontal distance from the polished rod to the balance point remains constant. Polished rod torque, on the walking beam, is the product of the vertical force and the horizontal distance, therefore the torque is constant. However, for walking beam mounted counter-weights, the distance from the walking beam balance point to the counterbalancing weights is constant. In that gravity works vertically through the weights, the horizontal distance from the weights to the balance point is constantly changing. Torque from the fixed point CBE is therefore at a minimum at the top of the stroke, a maximum in the middle, and a minimum again at the bottom. Therefore the torque from the chosen CBE will vary and only match at one position of the stroke. Counter-weights are typically placed to match the load in the middle of the stroke. The CBE mismatch increases as the walking beam angle is increased to obtain longer strokes.
As discussed above, the geometry of these units prohibits an ideal CBE throughout the entire stroke cycle, and an ideal CBE can only be realized at a few specific stoke positions. Consequently, the CBE is less than ideal at the other stroke positions. This limitation of the known methods for achieving CBE has required that the sizing of the various components of the pumping units, including walking beams, structural members, and gear boxes be sized for loading which occurs in the stroke positions where the ideal CBE cannot be achieved.
The known method of achieving CBE for walking beam pumping units is by utilizing counter-balance weights on either the walking beam opposite the polished rod or on the crank arms of the pumping unit. However, the mounting of weights on the walking beam greatly increases the load supported by the walking beam and its structure. This loading is compounded by the dynamics of stabilizing a large reciprocating mass at the end of the walking beam. These forces increase with longer strokes, rapid strokes, and larger loads. Therefore beam mounted counter-weights have been limited to the smallest of pumping units.
Crank mounted counter-weights have been utilized in an effort to overcome the above problems. However, while placing weights on the crank arms generally solves the dynamic load problems associated with the beam mounted weights, the crank mounted weights introduces a severe problem with matching CBE to polished rod load. Pumping units having crank mounted weights retain the problems from the horsehead geometry discussed above and add another compounding effect. As stated above, the induced torque from the fixed mounting point changes depending upon the stroke position (i.e., the walking beam angle). However, with crank mounted counter-weights, the induced CBE varies with the crank angle. Gravity works vertically through the center of gravity of the weights. Therefore the resulting CBE torque is at a maximum when the weights are horizontal, typically in the middle of the stroke. However the CBE goes to zero as the weights becomes vertical at the top and bottom of the stroke. Therefore, gear boxes for pumping units are typically sized one or two sizes larger than would otherwise be required.
Various attempts to solve this problem have been attempted. For example, U.S. Pat. No. 4,321,837 (Grigsby) discloses a system that uses additional moving crank mounted weights. This solution moves the zero point of CBE to a different crank angle which does not solve the wide variation in CBE. However, auxiliary weights rotating around a rotating crank arm also induce a costly complexity that has not proven cost effective.
As another potential solution to the problem, air balanced walking beam pumping units have been deployed. These units utilize the compression of a trapped gas, normally air, within a cylinder, to induce a CBE. The cylinders are typically installed on the horse head side of the walking beam. However, because the cylinder is attached to the walking beam at a fixed point, these units nevertheless share the geometry problems of units utilizing beam mounted counter-weights. The forces from the walking beam are applied to a fixed point while the effective length of the walking beam is constantly changing. Therefore the application of an ideal CBE does not match the tongue from the load.
In addition the pressure in the air cylinder is constantly changing as the cylinder expands or compresses the trapped air. These pressure swings can induce significant imbalance problems, which are attempted to be solved by eliminating pressure swings by venting air during compression and injecting compressed air during expansion. However, in operation, these units are generally not capable of producing a constant force CBE. In addition to these control problems, the air balanced units have proven very costly to operate. Operating costs and energy usage is much higher than for units utilizing mounted weights.
The above problems are based upon the loading induced at the polished rod by the pump actuation string. Another problem associated with actuation string loading is where insufficient rod load is realized at the subsurface pump to efficiently operate the pump because the actuation string is buckling. This problem is typically observed in wells which are highly deviated. In these installations, frictional loads induced on the actuation string in the deviated sections of the well may cause the actuation string to buckle and so reduce the rod weight realized at the subsurface pump that the plunger of the subsurface pump may not fully stroke within its barrel. In addition this buckling causes premature rod and tubing failures.